1. Field of the Invention
The present invention relates to a liquid crystal display, and particularly to a half-tone technology of a liquid crystal display.
2. Description of Related Art
When it comes to the wide-viewing-angle technology of liquid crystal display (LCD), a vertically aligned mode (VA mode) color LCD is considered as the most popular type at present. However, when a VA mode color LCD is viewed in an oblique direction, the skin color of Asians shown on the screen may tend to be blue or white. Such a phenomenon is called color wash-out. FIGS. 1A and 1B illustrate transmittance-voltage diagrams of a VA mode color LCD, wherein the vertical axis represents the transmittance and the horizontal axis represents the voltage applied. When the voltage increases, the transmittance of the perpendicular-viewing-angle curve 102 increases as well which shows a monotonic function, but the transmittance of the oblique-viewing-angle curve 104 is varied with a result that different gray-scale voltages have almost the same transmittance. This problem, which results in color wash-out, only occurs on VA mode color LCDs. In order to solve this problem, H. Yoshidaet et al. from Fujitsu Display Technologies Corporation have developed an improvement, which is to divide one pixel unit into two different gamma characteristic curves, so as to form two areas having different transmittance-voltage characteristics for color mixture. This method is called half-tone technology. Referring to FIG. 1B, the curve 106 is a transmittance-voltage curve having low threshold voltage and the curve 108 is a transmittance-voltage curve having high threshold voltage. These curves can be mixed to form a monotonic transmittance-voltage curve 110 for eliminating the color wash-out phenomenon.
Please refer to FIGS. 2A and 2B. At present, half-tone technology is mainly categorized into two types, CC type and TT type. FIG. 2A illustrates the CC type and FIG. 2B illustrates the TT type. The basic principle is to divide the original pixel unit into two areas, a first sub-pixel and a second sub-pixel, which include different gamma characteristic curves, so as to realize the afore-mentioned half-tone technology and eliminate the color wash-out phenomenon. FIG. 2C illustrates a gamma characteristic curve of the CC type, and FIG. 2D illustrates a gamma characteristic curve of the TT type. Referring to FIG. 2C for example, under a gray-scale voltage, a mixed gamma characteristic curve of a pixel unit is the sum of the gamma characteristic curves of the first sub-pixel and the second sub-pixel.
As shown in FIG. 2A, a pixel unit is divided into two areas, and a capacitance dividing method is applied to generating a sub-pixel capacitor 208 and a sub-pixel capacitor 214 which have different gamma characteristic curves. Herein, the voltage of the sub-pixel capacitor 208 is directly written by a data line through a transistor 202. The voltage of the sub-pixel capacitor 214 is determined after the voltage of the data line is divided by a serially connected storage capacitor 210. In other words, the sub-pixel capacitor 214 is in a floating status and the voltage is decided by the coupling method. The voltage of the sub-pixel capacitor 214 may be shifted when electric charges are captured during the operation of the panel, which may result in the problems such as low reliability, non-uniform image, and residual image.
Referring to FIG. 2B, a pixel unit is divided into two areas, and two different gamma characteristic curves are directly assigned from the system to the sub-pixel capacitor 226 and the sub-pixel capacitor 228 through transistors 218 and 220, two scan lines, or two data lines. This is the most direct method, but this method has the shortcomings of reducing the aperture ratio, complicating the system circuit (an additional gamma characteristic curve is required), doubling gate line driving or data line driving, increasing power consumption, and so forth.